Extreme droughts combined with heatwaves are intensifying in frequency and severity. The impacts on tree and forest functioning are manifold, and span from declining tree growth to reduced forest health and tree die-back as observed in many regions world-wide. These detriments have clear consequences for the well-documented contemporary carbon dioxide sink of forests and hence, their role in buffering climate change. Yet to date, we lack a comprehensive understanding to quantify long-term impacts on forest resilience and productivity beyond specific individual stress events. A striking knowledge-gap persists on what determines tree recovery and survival following drought release, including the thresholds causing functional damage and the role of repair mechanisms. Here, novel insights into physiological thresholds and post-stress recovery focusing on tree hydraulic processes and carbon metabolism are presented. A conceptual framework indicates that the persistence of stress legacy depends and on the degree of functional impairment, for instance hydraulic dysfunction or leaf senescence, and the ability for repair and regrowth. I argue that an improved physiological understanding of thresholds resulting in functional damage and how fast trees can repair and/or regrow tissues provides a promising avenue in order to integrate stress legacy into forest ecosystem models.

How to cite: Ruehr, N.: Capturing stress legacy: From tree physiology to forest resilience  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5564, https://doi.org/10.5194/egusphere-egu22-5564, 2022.

Climate change is expected to increase the frequency and intensity of extreme weather events like heat waves (HW), with potential negative impacts on grapevine productivity and wine quality in several regions. High temperatures during heat waves are usually accompanied by decreases in soil moisture, but few studies have explored the single and combined effects of temperature and drought on grapevine physiology. Using fully controlled environmental chambers we simulated a 6-day heat wave. Each chamber was assigned to one temperature treatment (control or heat). Inside each chamber, four grapevine plants (cv. Sauvignon b. on SO4 rootstock) were placed on weighing lysimeters. Half of the plants in each chamber were well-watered (watered plants) and the other half were subjected to drought (dry plants). Radiation intensity and air temperature mimicked average summer conditions near Bolzano (Italy). In the control chambers, the maximum daily temperature (Tmax) was 30°C for the entire period. In the heat chambers, Tmax reached 40° C on day 6, then it decreased on day 12 to 30° C.  Instantaneous leaf photosynthesis (Pn) as well as chlorophyll fluorescence parameters (Fv/Fm, Y(NPQ), qN, qP and qL) were measured manually in the late morning (10:00-12:00) and in the afternoon (15:00-17:00), together with stem water potential (SWP; 15:00-17:00). Plant transpiration (T), soil water potential and leaf temperature were continuously monitored throughout the experiment. During the HW, well-watered plants showed a marked reduction of Pn from the morning to the afternoon, which was not visible in the vines under control temperature. Drought significantly reduced Pn and, when combined with the heat stress, further decreased Pn with respect to the control temperature, both in the morning and in the afternoon. Daily T of watered plants during the HW was about 50% higher than T under the control temperature, while the HW had no effect on T of dry plants. Regardless of the chamber temperature, progressive drought caused stomatal closure, which in turn prevented stem water potential from reaching low levels (all SWP values were >-1.45 MPa). This, in turn, caused an increase of canopy temperature, which in the heated chambers reached a peak of around 45°C (up to 5°C warmer than in the watered plants). The heat stress decreased Fv/Fm both in watered and in dry plants, an effect that was stronger in the afternoon, but also occurred in the morning if the exposure to heat stress was prolonged. Other fluorescence parameters, like Y(NPQ) and qN, were affected by heat stress, while qP and qL were affected mainly by drought. At the end of the HW, chlorophyll fluorescence parameters recovered in watered plants, but not in the dry ones. Grapevines of the cv. Sauvignon b. were able to cope with a 6-day heat wave provided enough soil water was available to support the significantly enhanced transpiration rate. Drought had adverse effects on Pn regardless of the air temperature, but the drought-stressed plants that were also exposed to the heat wave experienced excess canopy temperatures. Leaf chlorophyll fluorescence proved to be a reliable indicator of heat stress.

How to cite: Asensio, D., Shtai, W., Kadison, A., Schwarz, M., Hoellrigl, J., Steiner, M., Raifer, B., Andreotti, C., Hammerle, A., Zanotelli, D., Hass, F., Niedrist, G., Wohlfahrt, G., and Tagliavini, M.: Interactive effects of high temperatures and drought on grapevine physiology during a simulated heat wave, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3171, https://doi.org/10.5194/egusphere-egu22-3171, 2022.

Limitation of photosynthesis by soil moisture variability is increasingly understood to be a central factor in the land carbon balance. Soil moisture dynamics are thought to be responsible for the bulk of interannual variability in the land carbon sink as well as the uncertainty in predicting it.

A key challenge in understanding this process is the mismatch between the spatial scales of cause and effect. Sharp and localised gradients in soil moisture availability can develop during drought, presenting a difficulty both for plants and researchers. Soil moisture heterogeneity triggers nonlinearities in flow in both the soil and plant, whose effects are not captured by current earth system models (ESM).

Most descriptions of soil moisture limitation of plants at earth system scales rely on a heuristic macro-scale ‘stress factor’ formulation that fails to adequately reflect current understanding of the inherently small-scale process. Recently, some models have adopted a linearised formulation of bulk flows in response to water potential gradients. While this step theoretically improves mechanistic representation of the process, increased prediction errors persist when vertical differences in soil moisture emerge while horizontal heterogeneity is commonly not represented at all.

This presentation introduces the upscaling approach to addressing this knowledge gap, which seeks new formulations of soil-plant hydrodynamics that bridge the scales of cause and effect. This approach relies on describing soil-plant hydrodynamics from first principles at small scale and mathematically scaling up the resulting formulations: deriving simple bulk scale forms while introducing as few approximations or errors as possible. Recent advances in this line of research include a fully general algorithm for upscaling the nonlinear equations describing flows in the root system without introducing discretisation error or increasing computational cost of finding solutions. Ongoing work aims to explore opportunities for addressing nonlinearities in the soil that arise from conceptual advances achieved in upscaling the plant flows. Applying the outputs of this promising theoretical approach in earth system models will require future empirical work to constrain the parameters of the new models at multiple scales.

How to cite: Bouda, M., Vandergborght, J., Couvreur, V., and Javaux, M.: Predicting plant water limitation in heterogeneously drying soils: the upscaling approach to improving soil-plant hydrodynamics in ESMs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12601, https://doi.org/10.5194/egusphere-egu22-12601, 2022.

The water-use efficiency (WUE, carbon assimilation per unit of water-use) describes a major axis of variability of ecosystems and identifies how these are coping with environmental changes. However, the response of WUE to climate change and hydrological extremes between different ecosystems remains poorly understood. 

Here we investigated how the WUE of ecosystems in Europe varied from 1995 - 2018, as long-term trends and in response to precipitation (P) and soil moisture (SM) droughts. We aggregated data from remote-sensing and reanalysis to calculate three different WUE indices, conducted Mann-Kendall trend analyses and determined WUE anomalies for different hydro-climates and plant functional types during P and SM deficits. Finally, we applied the Peter & Clark Momentary Conditional Independence (PCMCI) algorithm to identify causative networks of environmental variables and WUE and differences among ecosystems.

We found extensive, negative long-term WUE trends in Eastern Europe, where WUE is predominantly controlled by carbon assimilation (GPP). Further, we identified soil moisture and transpiration control of GPP as drivers for the positive WUE response to droughts in arid ecosystems. In contrast, negative trends in humid ecosystems were driven mostly by temperature, which governed GPP variability.

In addition, outputs from a state-of-the-art land-surface and carbon model (CLM5-BGC) will be used to compare trends, drought response and the causative relationships with the ones from satellite and reanalysis data in order to evaluate the model representation of ecosystem variability.

How to cite: Poppe Teran, C., Naz, B., Baatz, R., Hendricks-Franssen, H.-J., and Vereecken, H.: Causes of Water-Use Efficiency Variability in Europe and Their Representation in the Community Land Model v5, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4956, https://doi.org/10.5194/egusphere-egu22-4956, 2022.

Efficient leaf-scale temperature control is instrumental to vegetation functioning and ecosystem-scale resilience to a drying and warming climate. When evaporative cooling is suppressed during drought, leaf thermal regulation requires modulation of the leaf energy budget and the balance between sensible heat (H) and latent heat (LE) fluxes.

We obtained rare leaf energy budgets under field conditions by combining measurements using a new methodology and theoretical estimates in naturally droughted and artificially irrigated plots of a dry Mediterranean pine forest, with low and high evapotranspiration rates, respectively.

The measurements revealed that under the same radiative load, leaf cooling shifted from equal contributions to heat dissipation of H and LE in irrigated trees to almost exclusively through H in droughted ones while maintaining comparable leaf-to-air temperature differences.

The results demonstrate that an assessment of the leaf energy budget in the field provides the means to identify effective leaf temperature control in pine trees under drought, enhancing their resilience to current drying trends. The shift from LE to H provides an ‘air cooling’ mechanism that equals the efficiency of evaporative cooling. It also provides a leaf-scale basis for the large ecosystem-scale ‘convector effect’ identified in semi-arid forests.

How to cite: Muller, J. D., Rotenberg, E., Tatarinov, F., and Yakir, D.: Shift to efficient leaf cooling through sensible heat revealed by detailed energy budget in mature pine trees in drought manipulation field experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13247, https://doi.org/10.5194/egusphere-egu22-13247, 2022.

Recent research highlights the ability of large trees to maintain transpiration during tree water stress, for example in summer dry periods, by using stored plant water held in the relaxed xylem. Use of soil water at different depths by shallow and deep roots according to soil water content may also vary by species and seasonal climatic conditions to maintain transpiration demands. Here we present daylight tree water usage measurements of mature oaks (Quercus robur L.) under future-forest CO₂ conditions, derived from compensation heat pulse (HPC) xylem sap flux transducers deployed at Birmingham Institute of Forest Research (BIFoR) Free-Air CO₂ Enrichment (FACE) forest in Staffordshire UK. We experienced longer dry periods in summers 2018 and 2019 and variable annual summer precipitation overall.  Xylem sap flux data is collected half-hourly from eighteen oak trees and a smaller number of subdominant trees (Acer pseudoplatanus and Crataegus monogyna) in nine experimental patches: 3 patches with elevated CO₂ infrastructure (eCO₂); 3 with infrastructure but ambient CO₂ (aCO₂); and 3 ‘ghosts’ (no treatment, no infrastructure).  Each tree has two probesets, E and W facing; long (7 cm 4-sensor) probes in oak and short (4cm 2-sensor) probes, to accommodate the smaller diameter subdominant trees, are deployed. We compare individual tree responses under the three treatments across the leaf-on seasons for early years of the FACE project 2017–2021. Between-individual within-species variability of summertime monthly average of daily daylight water usage in oak is linearly proportional to tree stem radius (ca. 2.9 litres per millimetre radius, range; 274mm ≥ radius ≤ 465 mm) at the point of probeset insertion ca. 1.1–1.3 m above ground level and oak responds sub-daily to solar radiation reduction events during cloud cover. Diurnal stem sap flux responses to canopy photosynthetic demand typically exhibit increased sap flux from dawn to around midday (UTC) and symmetrical decrease to dusk. We describe our continuing investigation of soil-plant-atmosphere flows, monitoring root-xylem-stomatal changes, and discuss how these results, from our tree-centred forest view, provide valuable new perspectives and help to improve our understanding of future-forest-water use at larger scales, contributing to development of more realistic ecohydrological vegetation, soil and landscape models.

How to cite: Quick, S., Curioni, G., Krause, S., and MacKenzie, A. Rob.: Xylem sap dynamics of 175-year-old Quercus robur under elevated CO2 at BIFoR FACE, UK, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7255, https://doi.org/10.5194/egusphere-egu22-7255, 2022.

Increasing atmospheric CO₂ concentration influences the carbon assimilation rate of plants and stomatal conductance, and consequently affects the global cycles of carbon and water. However, the extent to which these physiological effects of increasing CO₂ significantly alters the land-atmosphere exchange of carbon and water is unclear.

To address this issue, we apply a comprehensive process-based land surface model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system) to study the propagation of effects of increasing atmospheric CO₂ concentrations into the carbon and water cycles. We analyze century-long simulations using factorial combinations of historical forcings for representative ecosystems across different climate regimes and biomes. We develop a statistical method based on the signal-to-noise ratio to detect the emergence of the increasing CO₂ effects. We find that the signal in gross primary production (GPP) emerges at relatively small CO₂ increase (Δ[CO₂] ~ 20 ppm) since the starting point of the time period (i.e., 1901), especially at sites where the leaf area index (LAI) is relatively high. The CO₂ signal in the transpiration water flux (normalized to evaporative leaf area) emerges only at relatively high CO₂ increase (Δ[CO₂] >> 40 ppm), rooted in its high sensitivity to climate variability. In general, the increasing CO₂ effect is stronger when plant productivity is not strongly limited by climatic constraints, stronger in forest-dominated rather than in grass-dominated ecosystems. The water cycle is less susceptible to the increasing CO₂ effects, mainly due to the compensatory effects of increasing LAI and reduced transpiration at leaf level. Our results from model simulations indicate when and where we expect to detect physiological CO₂ effects in in-situ flux measurements. Finally, we apply the statistical methods to quantify the increasing CO₂ effects on carbon and water flux measurements across the FLUXNET network. Overall, the model-based analyses along with the observational study focused on the detection and potential quantification of iCO₂ effects, are critical and provide robust assessments of how the system will continue to change as CO₂ continues to rise.

How to cite: Zhan, C., Orth, R., Migliavacca, M., Zaehle, S., Reichstein, M., Engel, J., Rammig, A., and J. Winkler, A.: Emergence of the physiological effects of increasing CO2 in the land-atmosphere exchange of carbon and water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7273, https://doi.org/10.5194/egusphere-egu22-7273, 2022.

Dense forest canopy influences understory microclimate by lowering the air and surface temperatures. It can mitigate the adverse effects of global climate change in forest communities. On the other hand, low light availability in dense forests is the main constraint for the understory species photosynthesis. And the resulting lack of carbohydrates may make herbs more vulnerable to increasing drought induced by climate change. Here we assessed the survival and ecophysiology of two morphologically similar but physiologically rather contrasting perennial herbs typical of temperate forest understories: isohydric Asarum europaeum and relatively anisohydric Hepatica nobilis. We emulated a summer drought period by growing adult plants of these two species in the greenhouse under 10% (simulating sparse forest) and 1% (simulating dense forest canopy) of outdoor light. Half of the individuals were subjected to water stress by withholding the watering until ca 50% mortality occurred. After the drought phase, we fully watered the plants and let them recover for one month.

The lowest predawn water potential measured during the experiment was -3.6 MPa, and the midday water potentials of plants in the sun were -5.4 and -8.1 MPa for Asarum and Hepatica, respectively. Light saturated photosynthesis (A_sat) of fully watered herbs was by more than 50% higher in the Asarum than in Hepatica at the end of the experiment. The two herbs plastically adjusted their A_sat to the light environment, so that A_sat of shaded Hepatica was by 38% and of shaded Asarum by 29% lower than in controls. After the period of drought, A_sat of stressed plants of both species grown in the lighter conditions fully recovered. In the shaded variant, however, only A_sat of Hepatica recovered but that of Asarum did not. Intrinsic water use efficiency (WUE_i) was higher in isohydric Asarum than in Hepatica. WUE_i was also higher in the herbs grown in light than in the shade. Specific dyeing to functional xylem indicated a higher proportion of conductive xylem (which means less damage by embolism) in plants grown in shade than in the light, in both species. To sum up, sufficient light may help some understory species to recover from severe water stress.

How to cite: Urban, J., Matoušková, M., Plichta, R., Gebauer, R., Houšková, K., Vitásek, R., and Hédl, R.: The combined effect of light availability and drought on survival of two forest understory herb species, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7442, https://doi.org/10.5194/egusphere-egu22-7442, 2022.

Thawing and degradation of permafrost in high latitudes could have an important effect on the Earth’s greenhouse budget. Implications of increased nutrient availability resulting from thawing of nutrient-rich permafrost, however, remain poorly assessed, despite nutrients having being identified as a strong present-day constraint for plant growth and microbial activity in the high latitudes. In our pan-arctic scale study, we extend the terrestrial ecosystem model QUINCY, which already couples C-N-P cycles in soil and vegetation, for a better representation of high-latitude processes. With this model version, we perform historical simulations at the site-level over 1960-2019. Averaged over high-latitude grassland sites, our simulations show an average increase in the soil active layer depth of 0.1m and an increased gradient of biologically-available P and N at the permafrost front. In spite of this, only 11 % of the simulated increase over the GPP (+34%) is a result of increased nutrient supply from permafrost organic matter degradation. This owes to spatial and temporal decoupling of the simulated vegetation growth peak (mid-to-late-July), the time period where plant nutrient demand is the highest, and the maximum of the seasonal thaw depth (mid-to-late August), the time period in which nutrients in the deep active layer would potentially be available for uptake. As a result, increased nitrogen at the permafrost front and alternating aerobic-anaerobic conditions contribute to enhancing nitrification and denitrification in the model, causing a weak source of N₂O to the atmosphere of 0.7 kg N ha^-1 yr^-1, which undergoes a considerable upward trend of up to 0.1 kg N ha^-1 decade^-1, locally,over the simulation time frame. Considering the vastness of the permafrost domain, and that N₂O emissions from these regions have been largely neglected in the past, these results imply that high latitudes could be a considerable and growing contributor to the global atmospheric N₂O budget.

How to cite: Lacroix, F., Zaehle, S., Caldararu, S., Schaller, J., Stimmler, P., and Goeckede, M.: Temporal Disconnect of Seasonal Plant Nutrient Demand and Thaw Depth implies an Increasing Source of N2O in High-Latitude Permafrost Ecosystems , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7693, https://doi.org/10.5194/egusphere-egu22-7693, 2022.

Plants communicate information about their status, intra- and inter- plant, and with other ecosystem members, through the release of volatile organic compounds (VOCs). The effects of rising CO₂ in conjunction with ozone (O₃) on plant VOC emissions is not yet fully understood, but research suggests that some herbivore-induced VOCs are degraded by O₃, potentially reducing their signalling function. Furthermore, elevated CO₂ has been shown to attenuate induced VOC responses to herbivory in Brassica oleracea. 

We are using two tri-trophic model systems; black poplar [Populus nigra betulifolia], winter moth [Operophtera brumata] and a tachinid fly parasitoid of winter moth, Cyzenis albicans; and oil seed rape [Brassica napus], diamond back moth [Plutella xylostella] (DBM), and a parasitoid of DBM, braconid wasp, Cotesia plutella. Additionally, we are working within two ground-breaking facilities; the Birmingham Institute of Forest Research (BIFoR)’s free-air carbon enrichment (FACE) experiment, and University of Reading’s free-air diesel and ozone enrichment experiment. We will also collect some data from lab-based experiments.

Our project seeks to characterise the volatile organic compound (VOC) profiles emitted for both plants under herbivory, examine how these VOC profiles differ under combined elevated CO₂ and O₃, and explore whether changes to VOC profiles impact key ecological relationships, e.g, the ability of plants to signal to herbivore enemies.

How to cite: James, L., Pfrang, C., Girling, R., Hayward, S., and MacKenzie, R.: Characterising volatile organic compound emission changes in native black poplar under elevated carbon-dioxide (CO2), elevated ozone (O3) and herbivory, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4487, https://doi.org/10.5194/egusphere-egu22-4487, 2022.

The drought resilience of forest ecosystems depends on the water use strategies and the degree of vulnerability to hydraulic failure of individual tree species. The coordination between hydraulic and allocation traits along with stomatal control determines the tree water-use strategy, ranging from acquisitive to conservative tree species. This work explores the role of different plant hydraulic traits (Ψ_P50, c_k, and k_max in the Community Land Model 5.0) on the simulated plant water use dynamics. We selected two broadleaved tree species (Quercus ilex L. and Fagus sylvatica L.) at four SAPFLUXNET experimental sites having contrasting climate conditions. From the range of plant hydraulic traits reported for each species in the Xylem Functional Traits (XFT) database and other literature, the most vulnerable and most resistant parameter combination was chosen as extreme cases. Four sets of experiments were carried out that include modification of the shape of the plant vulnerability curve changing only Ψ_P50 and c_k (CS-experiment), changing only k_max (k-experiment), changing the three parameters of the vulnerability equation (FC-experiment), and changing gradually k_max (KS-experiment) to test the model sensitivity to k_max. The stand transpiration obtained from SAPFLUXNET was used as a benchmark for the model comparisons. The CS-experiment revealed that a vulnerable configuration increases the modeled transpiration during conditions with ample water supply, and causes severe water stress and reduced transpiration during dry periods as compared to a resistant configuration. This indicates that transpiration is hydraulically limited even at ample water supply in the model so that the more negative Ψ_P50 enables increased transpiration. Although a more negative Ψ_P50 allows the vegetation to access more soil water than would be the case for vulnerable configurations, the difference in actual plant available water is small at this dry end of the water retention curve, and hence the dry period water stress is mainly determined by early-season transpiration. The K- and KS- experiments illustrate the role of k_max to effectively scale up/down the transpiration response. Finally, the FC-experiments revealed the potential of plant hydraulic traits to mimic either conservative or acquisitive water-use strategies, allowing the vegetation to manage more efficiently the soil water resources. This work underlines the importance of selecting a suitable plant hydraulic parametrization contemplating the diversity of plant water use strategies.

How to cite: Jiménez-Rodríguez, C. D., Sulis, M., Mallick, K., and Schymanski, S.: On the role of intraspecific variability of plant hydraulic traits when modelling plant water use strategies at different forested sites in Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7199, https://doi.org/10.5194/egusphere-egu22-7199, 2022.